Field of the Invention
The present invention relates to polymeric materials comprising chalcogenide elements with organic crosslinking moieties.
Description of the Prior Art
Infrared (IR) optical technology has numerous potential applications in civil, medical, and military areas, where inorganic semiconductors (e.g., Ge and Si) and chalcogenide glasses based on sulfur, selenium, and tellurium, have been widely used as materials for device components due to their high refractive index (n˜2.0-4.0) and low optical losses from 1-10 μm. While such materials are well suited for these applications, they are inherently more expensive and difficult to process due to the high processing temperatures and volatile nature of the compounds in comparison to organic polymeric materials. Polymers are desirable due to their light weight, low cost and are often easily processed into optical components, where they are finding many applications for use in the visible region. However, the development of polymeric materials for short wave infrared (SWIR) and mid wave infrared (MWIR) optical applications has not been achieved due to challenges in designing systems with sufficiently high refractive index (n) and transparency in the infrared spectral region.
Organic polymers have refractive indices that are low, where values generally range between 1.3-1.7. (Brandrup et al., Polymer Handbook, 4th ed., John Wiley & Sons, New York, (2005)). Furthermore, because polymers are carbon and hydrogen based, they cannot be used for MWIR (3-5 μm) optical applications due to carbon-hydrogen bond absorptions in this region, for example the C—H stretch is found at 3000 cm−1 (3.33 μm). Therefore, this region cannot be used for devices where light transmission is required. Replacement of aliphatic C—H bonds with elements that impart changes to the reduced mass, such as C-D, C—Cl, and C—F units, substantially lowers the energy of fundamental bond vibration and the absorption bands are moved significantly further into IR region. However, simply fluorinating a polymer, while eliminating C—H absorption, will significantly lower its refractive index.
Refractive index and dispersion dictate the shape and size of lenses, and higher values are needed for better focusing power and wave-guiding of light. The most common way to increase the refractive index of an organic polymer is by the incorporation of highly polarizable species into either the backbone or as pendant groups. (Liub et al., “High refractive index polymers: fundamental research and practical applications,” J. Mater. Chem., 19, 8907-8919 (2009)). Sulfur, with a polarizability of 2.9 Å3 as compared to 1.8 Å3 for carbon, is the most common species used for increasing the refractive index. Selenium has a greater polarizability with a value of 3.8 Å3, and tellurium has an even greater polarizability than selenium with a value of 5.5 Å3. Therefore, chemically stable and easily processable polymers that are predominantly composed of a higher polarizability element such as sulfur, selenium and tellurium will provide an excellent opportunity to greatly increase and control the refractive index.
The chalcogenides sulfur and selenium exist predominantly as eight membered rings at room temperature. Upon heating past their melting points, chalcogenides will undergo a ring opening polymerization. These polymers are metastable however and will convert back to crystalline species upon cooling. Sulfur and selenium have been shown to interact and have complete liquid miscibility, forming a copolymer. (Berbenni et al., “A DSC characterization of sulphur-selenium interaction phenomenology,” Thermochimica Acta, 237, 253-260 (1994)). For both species the opened rings form chains terminated by radicals. These radicals then combine to form longer chains. The polymer can be stabilized by the addition of divinyllic crosslinking moieties to the heated melt. This method has been applied in the case of sulfur alone. (Pyun et al., High Sulfur Content Copolymers and Composite Materials and Electrochemical Cells and Optical Elements Using Them, (WO2013023216); Griebel et al., “New Infrared Transmitting Material via Inverse Vulcanization of Elemental Sulfur to Prepare High Refractive Index Polymers,” Adv. Mater., 26, 3014-3018 (2014); Chung et al., “The use of elemental sulfur as an alternative feedstock for polymeric materials,” Nature Chemistry Vol 5, 518 (2013); and Namnabat et al., “Sulfur copolymers for infrared optical imaging,” Proc. of SPIE Vol. 9070 90702H-1 (2014)). However, this technique alone is not able to incorporate Se into the backbone due to the different processing characteristics of pure Se and S.